The purpose of this research is to delineate barriers and pathways within the cochlea using ultramicroscopic investigation in conjunction with the following procedures: (1) mechanical alteration of anatomic barriers; (2) alterations in the normal state of the membraneous labyrinth using ototoxic antibiotics, diuretics, acoustic trauma, and propylthiouracil; (3) injection of electron-microscopically visible tracers, such as Horseradish Peroxidase, Thorotrast, Ferritin, Ruthenium Red, and Lanthanum; (4) use of (3) in altered circumstances of (1) and (2); (5) study of the normal ultrastructure of the membraneous labyrinth, in conjunction with efferent and afferent innervation following surgical transection of the Olivo-cochlear bundle. The object is to delineate the compartments of the membraneous labyrinth (endolymphatic, perilymphatic, vascular); i.e., the determination of which portions of the labyrinth belong to which compartments, and the types, amount, and pathways of exchange between these compartments in normal and altered states. Although the immediate aims are toward this basic knowledge, our ultimate goal is to determine the cause of two of the most common clinical conditions seen today: endolymphatic hydrops and sudden deafness.